Chronic obstructive pulmonary disease (COPD) includes conditions such as, e.g., chronic bronchitis and emphysema. COPD currently affects over 15 million people in the United States alone and is currently the third leading cause of death in the country. The primary cause of COPD is the inhalation of cigarette smoke, responsible for over 90% of COPD cases. The economic and social burden of the disease is substantial and is increasing.
Chronic bronchitis is characterized by chronic cough with sputum production. Due to airway inflammation, mucus hypersecretion, airway hyperresponsiveness, and eventual fibrosis of the airway walls, significant airflow and gas exchange limitations result.
Emphysema is characterized by the destruction of the lung parenchyma. This destruction of the lung parenchyma leads to a loss of elastic recoil and tethering which maintains airway patency. Because bronchioles are not supported by cartilage like the larger airways, they have little intrinsic support and therefore are susceptible to collapse when destruction of tethering occurs, particularly during exhalation.
Acute exacerbations of COPD (AECOPD) often require emergency care and inpatient hospital care. An AECOPD is defined by a sudden worsening of symptoms (e.g., increase in or onset of cough, wheeze, and sputum changes) that typically last for several days, but can persist for weeks. An AECOPD is typically triggered by a bacterial infection, viral infection, or pollutants, which manifest quickly into airway inflammation, mucus hypersecretion, and bronchoconstriction, causing significant airway restriction.
Despite relatively efficacious drugs (long-acting muscarinic antagonists, long-acting beta agonists, corticosteroids, and antibiotics) that treat COPD symptoms, a particular segment of patients known as “frequent exacerbators” often visit the emergency room and hospital with exacerbations and also have a more rapid decline in lung function, poorer quality of life, and a greater mortality risk.
Reversible obstructive pulmonary disease includes asthma and reversible aspects of COPD. Asthma is a disease in which bronchoconstriction, excessive mucus production, and inflammation and swelling of airways occur, causing widespread but variable airflow obstruction thereby making it difficult for the asthma sufferer to breathe. Asthma is further characterized by acute episodes of airway narrowing via contraction of hyper-responsive airway smooth muscle.
The reversible aspects of COPD include excessive mucus production and partial airway occlusion, airway narrowing secondary to smooth muscle contraction, and bronchial wall edema and inflation of the airways. Usually, there is a general increase in bulk (hypertrophy) of the large bronchi and chronic inflammatory changes in the small airways. Excessive amounts of mucus are found in the airways, and semisolid plugs of mucus may occlude some small bronchi. Also, the small airways are narrowed and show inflammatory changes.
In asthma, chronic inflammatory processes in the airway play a central role in increasing the resistance to airflow within the lungs. Many cells and cellular elements are involved in the inflammatory process including, but not limited to, mast cells, eosinophils, T lymphocytes, neutrophils, epithelial cells, and even airway smooth muscle itself. The reactions of these cells result in an associated increase in sensitivity and hyperresponsiveness of the airway smooth muscle cells lining the airways to particular stimuli.
The chronic nature of asthma can also lead to remodeling of the airway wall (i.e., structural changes such as airway wall thickening or chronic edema) that can further affect the function of the airway wall and influence airway hyper-responsiveness. Epithelial denudation exposes the underlying tissue to substances that would not normally otherwise contact the underlying tissue, further reinforcing the cycle of cellular damage and inflammatory response.
In susceptible individuals, asthma symptoms include recurrent episodes of shortness of breath (dyspnea), wheezing, chest tightness, and cough. Currently, asthma is managed by a combination of stimulus avoidance and pharmacology.
The autonomic nervous system (ANS) provides constant control over airway smooth muscle, secretory cells, and vasculature. The ANS is divided into two subsystems, the parasympathetic nervous system and the sympathetic nervous system. These two systems operate independently for some functions, and cooperatively for other functions. The parasympathetic system is responsible for the unconscious regulation of internal organs and glands. In particular, the parasympathetic system is responsible for sexual arousal, salivation, lacrimation, urination, and digestion, among other functions. The sympathetic nervous system is responsible for stimulating activities associated with the fight-or-flight response. Although both sympathetic and parasympathetic branches of the ANS innervate lung airways, it is the parasympathetic branch that dominates with respect to control of airway smooth muscle, bronchial blood flow, and mucus secretions.
FIG. 1 illustrates the cholinergic control of airway smooth muscle and submucosal glands. An airway 100 may include an inner surface 102 that includes epithelial tissue 104. A nerve fiber 106 may include a plurality of receptors 108 that are disposed within epithelial tissue 104. Nerve fibers 106 may be C-fibers having receptors 108 disposed within epithelial tissue 104. Nerve fibers 106 may be afferent (sensory) nerves that carry nerve impulses from receptors 108 toward central nervous system (CNS) 109. Receptors 108 may respond to a wide variety of chemical stimuli and other irritants, such as, e.g., cigarette smoke, histamine, bradykinin, capsaicin, allergens, and pollens. C-fibers can also be triggered by autocoids that are released upon damage to tissues of the lung. The stimulation of receptors 108 by the various stimuli elicits reflex cholinergic bronchoconstriction.
Parasympathetic innervation of the airways is carried exclusively by vagus nerve 110 (e.g., the right and left vagus nerves). Upon receiving a signal from nerve fiber 106, CNS 109 may send a signal to initiate bronchoconstriction and/or mucus secretion. Cholinergic nerve fibers (e.g., nerve fibers that use acetylcholine (ACh) 120 as their neurotransmitter) arise in the nucleus ambiguous in the brain stem and travel down a vagus nerve 110 (right and left vagus nerves) and synapse in parasympathetic ganglia 112 which are located within the airway wall. These parasympathetic ganglia are most numerous in the trachea and mainstem bronchi, especially near the hilus and points of bifurcations, with fewer ganglia that are smaller in size dispersed in distal airways. From these ganglia, short post-ganglionic fibers 114 travel to airway smooth muscle 116 and submucosal glands 118. Ach 120, the parasympathetic neurotransmitter, is released from post-ganglionic fibers and acts upon M1- and M3-receptors on smooth muscles 116 and submucosal glands 118 to cause bronchoconstriction (via constriction of smooth muscles 116), and the secretion of mucus 122 within airway 100 by submucosal glands 118, respectively. Ach 120 may additionally regulate airway inflammation and airway remodeling, and may contribute significantly to the pathophysiology of obstructive airway diseases. Thus, fibers 114 may be efferent fibers (motor or effector neurons) that are configured to carry nerve impulses away from CNS 109.
FIG. 2 illustrates additional afferent nerve fibers located in airway 100 and in airway smooth muscle 116. Airway 100 may include one or more nerve fibers 106 and receptors 108 as described with reference to FIG. 1. Additionally, one or more nerve fibers 206 having one or more receptors 208 may be disposed within epithelial tissue 104. Nerve fibers 206 may be myelinated Rapidly Adapting Receptors (RAR) that respond to mechanical stimuli and are responsible in part for bronchoconstriction. Receptors 208 may respond to mechanical stimuli such as, e.g., water, airborne particulates, mucus, and the stretching of the lung during breathing or coughing. RARs may cause bronchoconstriction and are triggered by mechano-stimulation (e.g., mechanical pressure or distortion) and/or chemo-stimulation. Additionally, RARs may be triggered secondary to bronchoconstriction, leading to an amplification of the constriction response.
Airway smooth muscle 116 may be coupled to one or more receptors 210. Receptors 210 may be, e.g., Slowly Adapting Receptors (SARs) that are coupled to one or more nerve fibers 211.
Bronchial hyperresponsivity (BHR) may be present in a considerable number of COPD patients. Various reports have suggested BHR to be present in between ˜60% and 94% of COPD patients. This “hyperresponsivity” could be due to a “hyperreflexivity.” However there are several logical mechanisms by which parasympathetic drive may be overactivated in inflammatory disease. First, inflammation is commonly associated with overt activation and increases in excitability of vagal C-fibers in the airways that could increase reflex parasympathetic tone. Secondly, airway inflammation and inflammatory mediators have been found to increase synaptic efficacy and decrease action potential accommodation in bronchial parasympathetic ganglia, effects that would likely reduce their filtering function and lead to prolonged excitation. Thirdly, airway inflammation has also been found to inhibit muscarinic M2 receptor-mediated auto-inhibition of ACh release from postganglionic nerve terminals. This would lead to a larger end-organ response (e.g., smooth muscle contraction) per a given amount of action potential discharge. Fourthly, airway inflammation has been associated with phenotypic changes in the parasympathetic nervous system that could affect the balance of cholinergic contractile versus non-adrenergic non-cholinergic (NANC) relaxant innervation of smooth muscle.
Because airway resistance varies inversely with the fourth power of the airway radius, BHR is believed to be a function of both bronchoconstriction and inflammation. Inflammation in the airway walls reduces the inner diameter (or radius) of the airway lumen, thus amplifying the effect of even baseline cholinergic tone, because for a given change in muscle contraction, the airway lumen will close to a greater extent. BHR is likely caused by hypersensitivity of receptor nerve fibers, such as, e.g., C-fibers, RAR fibers, SAR fibers, and the like, lower thresholds for reflex action initiation, and reduced self-limitation of acetylcholine release.
The majority of vagal afferent nerves in the lungs are nociceptors that are adept at sensing the type of tissue injury and inflammation that occurs in the lungs in COPD. In addition, stretch sensitive afferent nerves are present in the lungs and can be activated by the tissue distention that occurs during eupneic (normal) breathing. The pattern of action potential discharge in these fibers depends on the rate and depth of breathing, the lung volume at which respiration is occurring, and the compliance of the lungs. Therefore, because COPD patients exhibit impaired breathing, the activity of nociceptive and mechano-sensitive afferent nerves is grossly altered in patients with COPD. The distortion in vagal afferent nerve activity in COPD may lead to situations where these responses are out of sync with the body's needs.
Thus, a need exists for patients suffering from diseases of the lung.